I have to agree with screamer. By the time you save enough money on gas to pay for the swap; you'll be ready for a new ride anyway (unless you already have the motor in hand). Also, if you use the truck to haul anything, you will miss the hell out of that 454. There is no way you will pick up a 350 that puts out the kind of torque your 454 already does for the $1000.00 junkyard motor that screamer mentioned.

Anyway, if you do get one out of junkyard, make sure it is a carb to oil pan motor (may as well get the exhaust manifolds as well) and not just a long block. Otherwise you are looking at buying a new intake manifold and exhaust manifolds/headers and possibly having to modify the exhaust system beyond that to make 'em fit. Then all ya gotta do is hope the motor you get from the junkyard lasts.

One final thought and I may be way off on this one, so please forgive me if I am, as I'm not trying to offend you; but, I can only assume that since you are asking the question, you wont actually be the one doing the work to change the motor over. If that's the case, you can add labor to your bills as well.

Anyway, I do wish you the best of luck with it.

Cheers,
Firechicken